Emissions legislation has resulted in the fitting of exhaust gas aftertreatment systems, in particular the fitting of a diesel particle filter (DPF) to the exhaust system of many diesel engined motor vehicles. Such filters trap small carbonaceous particles of an exhaust gas stream, to prevent emission to atmosphere. Over time, such filters become progressively more loaded so that the soot mass increases towards a design maximum, and regeneration is required.
Regeneration is accomplished by oxidizing the soot mass. Active regeneration requires the exhaust gas temperature at the inlet of the DPF to be raised to about 600° C., and this is generally accomplished by introducing unburned fuel into the engine after normal combustion (so called post-injection). The unburned fuel is combusted in the usual catalytic converter, and thereby raises the temperature of the exhaust gas so that regeneration of the DPF can occur spontaneously. Active regeneration is initiated by appropriate management of engine fuelling.
Passive regeneration may occur spontaneously if the exhaust stream has a high nitrogen dioxide flux, for example when an engine has a high NOx exhaust. Such passive regeneration occurs at a lower exhaust gas temperature, of about 400° C.
An active regeneration may take around 10 minutes, and a passive regeneration about 30 minutes. It will be understood however that the regeneration time is influenced by the volume of the DPF, the soot mass contained therein, exhaust gas conditions and other relevant factors, so that the times quoted are merely illustrative.
In order to minimize post-injection of fuel, which has the disadvantages of increasing fuel consumption and contaminating engine lubricant, active regeneration is most desirable in highway driving where engine speed is high and exhaust gas temperature may also be assumed to be high. In off-road driving or town driving, some regeneration strategies may not successfully complete regeneration because exhaust gas temperature at the DPF remains below a threshold, and accordingly the DPF may become blocked over an extended period of this kind of driving.
Passive regeneration could allow regeneration at lower temperatures, but typically requires forcing the vehicle engine to operate with a high NOx output, which is undesirable because of the inevitable increase in noxious emissions, or by providing a separate source of NO2, which is generally impractical.
It is against this background that the present invention has been conceived. Embodiments of the invention may provide a method or a vehicle which adopts a regeneration strategy that can best provide appropriate regeneration in a range of vehicle operating conditions. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.